Process for producing water-absorbing resin

ABSTRACT

The disclosed process for producing a water-absorbing resin comprising: a polymerization step of polymerizing an aqueous unsaturated monomer; a drying step of drying a particulated water-containing gel-liked crosslinked polymer, obtained in a finely crushing step during the polymerization or after the polymerization; a pulverizing step after the drying, a classification step after the drying, and a surface treatment step after the classification. In the process is, the surface-treatment step is interrupted with a heating treatment apparatus kept in a heated state and thereafter the surface treatment step is restarted.

TECHNICAL FIELD

The present invention relates to a process for producing awater-absorbing resin. More concretely, the present invention relates toa method for providing a water-absorbing resin with high physicalproperties by uniform surface cross-linking in a process forcontinuously producing a water-absorbing resin in a huge scale involvingjoined continuous steps.

BACKGROUND ART

A water-absorbing resin (SAP/Super Absorbent Polymer) is awater-swelling and water-insoluble polymer gelling agent and has beenused for mainly disposable uses including absorbing articles such aspaper diapers and sanitary napkins, as well as water retention agentsfor agriculture and horticulture, water-stopping materials forindustrial use, and the like. A large number of monomers and hydrophilicpolymers have been proposed as raw materials for such a water-absorbingresin and particularly, poly(acrylic acid (salt))-type water-absorbingresins using acrylic acid and/or a salt thereof as a monomer have beenindustrially used most frequently owing to their high water absorbentcapability.

Such a water-absorbing resin can be obtained by finely crushing awater-containing gel-like polymer, which is obtained by polymerizing anaqueous monomer solution, during or after the polymerization, and dryingthe obtained particulated water-containing gel-like crosslinked polymer.After drying, if necessary, a pulverizing step and a classification stepare carried out and arbitrarily, the obtained product is subjected tosurface-crosslinking before drying or after drying. One or more stepssuch as a fine powder recovery step, an undried matter removal stepafter drying, a packaging step, and an addition step of adding otheradditives (fine particles, a deodorant, an antibacterial agent, and thelike) may also be carried out arbitrarily. A method to be employed as acommon polymerization method is aqueous solution polymerization orreverse-phase suspension polymerization and the product form isgenerally a powder of about 10 to 1000 μm. Such a process for producinga water-absorbing resin involving many steps is exemplified in PatentDocuments 1 to 13 or the like.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: WO No. 2009/113679 pamphlet-   Patent Document 2: WO No. 2009/113678 pamphlet-   Patent Document 3: WO No. 2009/113671 pamphlet-   Patent Document 4: WO No. 2009/113672 pamphlet-   Patent Document 5: WO No. 2009/119754 pamphlet-   Patent Document 6: WO No. 2009/123197 pamphlet-   Patent Document 7: U.S. Pat. No. 6,716,894-   Patent Document 8: U.S. Pat. No. 6,727,345-   Patent Document 9: U.S. Pat. No. 6,164,455-   Patent Document 10: U.S. Pat. No. 6,817,557-   Patent Document 11: U.S. Pat. No. 6,641,064-   Patent Document 12: U.S. Pat. No. 6,291,635-   Patent Document 13: EP Patent No. 1949011

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In recent years, because of increase of the demand for paper diapers,and the like, production scale of the water-absorbing resin has beenbecoming wider and it leads to a tendency of scale up for one line ofproduction apparatus and increase of polymerization concentration(increase of concentration of an aqueous monomer solution, e.g.,disclosed in Patent Document 7). In order to satisfy various high levelsof requests from users, a wide variety of products of water-absorbingresins have been developed. Accordingly, in the present situation, thereare many steps and many additives employed after the drying step: forexample, a pulverizing step, a classification step, a surface treatmentstep (particularly, a surface-crosslinking step), a conveying step, agranulation step, an additive addition step, a fine powder recovery stepand the like, according to the above-mentioned patent documents and thelike.

Therefore, the present situation is that a lot of models are producedthrough many steps in a single plant. The respective steps are sometimescarried out in a batch method, but in general, the main stream is acontinuous process. Even if some steps are carried out in a batchmethod, the process in which the respective steps are joined is as awhole process a substantially continuous process for production. Inaddition, substantially continuous means that even in the case of batchsteps, the steps are repeatedly continuously carried out. For example,it means a state where the hydrogel or a dried material thereof issubjected to a storage step after repeated batch steps to becontinuously supplied to continuous steps (even if some batch steps areinvolved) and these steps are regarded as continuous as a whole process.

In a process for producing a water-absorbing resin which involves manysteps, the continuous flow of a water-absorbing resin or a hydrogelthereof sometimes stops in a production plant because of periodicmaintenance or a temporary trouble (an operation trouble in some steps).Patent Documents 9 to 12 disclose such a trouble (stop of operation) ina crushing step, a conveying step, or a storage step.

In the case of joined steps, if even a few steps are stopped, the entireplant in which the steps are joined has to be stopped and, therefore,there occurs problems that, at the time of restarting, the quality isnot stable, excess energy is required for restarting the operation, anexcess load is applied to an apparatus (e.g., it can be confirmed byincrease of the power consumption) and the like, and in the worst case,the operation may be stopped. In addition, a colored foreign matter (adiscolored material of mainly a water-absorbing resin) may contaminate awater-absorbing resin at the time the operation is restarted.

Solutions to the Problems

To solve the above-mentioned problems, the inventors of the presentinvention have made earnest investigations and have finally found thatthere is a problem in the surface treatment step and that theabove-mentioned problems can be solved by controlling a stopping methodfor the surface treatment step.

That is, the present invention (the first invention) provide a processfor producing a water-absorbing resin comprising a polymerization stepof polymerizing an aqueous unsaturated monomer solution, a drying stepof drying a particulated water-containing gel-like crosslinked polymerobtained in a crushing step during the polymerization or after thepolymerization, a pulverizing step after the drying, a classificationstep after the drying, and a surface treatment step for thewater-absorbing resin powder after the classification step, wherein thesurface treatment step is interrupted with a heating treatment apparatuskept in a heated state and thereafter the surface treatment step isrestarted. Provide that, the interruption of the surface treatment stepmeans a state where the water-absorbing resin powder is substantiallyabsent in the heating treatment apparatus, or not charged to or notdischarged out of the heating treatment apparatus in the continuoussurface treatment.

To solve the above-mentioned problems, the present invention (the secondinvention) provide a process for producing a water-absorbing resincomprising a polymerization step of polymerizing an aqueous unsaturatedmonomer solution, a drying step of drying a particulatedwater-containing gel-like crosslinked polymer obtained in a crushingstep during the polymerization or after the polymerization, apulverizing step after the drying, a classification step after thedrying, and a surface treatment step for the water-absorbing resinpowder after the classification step, wherein the surface treatment stepis interrupted and heating of a heating treatment apparatus is stopped,then cleaning of the inside of the heating treatment apparatus isstarted within 100 hours and thereafter the surface treatment step isrestarted. Provide that, the interruption of the surface treatment stepmeans a state where the water-absorbing resin powder is substantiallyabsent in the heating treatment apparatus, or is not charged to or notdischarged out of the heating treatment apparatus in the continuoussurface treatment.

Effects of the Invention

In continuous production of the water-absorbing resin, particularly, thecontinuous surface treatment step of carrying out a surface treatmentfor not less than 1 ton per hour, the water-absorbing resin can bestably produced by stably carrying out the surface treatment withoutcoloration.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view showing the typical continuous productionflow of a water-absorbing resin.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a method for producing the water-absorbing resin of thepresent invention will be described in detail; however, the scope of thepresent invention is not restricted to the following description, andthose other than the following examples can be properly modified andcarried out in a range where the gist of the present invention is notimpaired. Specifically, the present invention is not limited to each ofthe following embodiments, and various modifications can be made withina range shown by the claims and embodiments carried out by properlycombining each technical means disclosed with different embodiments arealso included within the technical scope of the present invention.

[1] DEFINITION OF TERMS

(1-1) Water-Absorbing Resin

In the present invention, the “water-absorbing resin” means awater-swelling and water-insoluble polymer gelling agent. The“water-swelling” means that CRC (absorption capacity without load)defined in ERT 441.2-02 is indispensably 5 [g/g] or more and the“water-insoluble” means that Ext (extractables) defined in ERT 470.2-02is indispensably 0 to 50 mass %.

The water-absorbing resin can be properly designed in accordance withthe use and is not particularly limited, and is preferably a hydrophiliccrosslinked polymer obtained by crosslinking polymerization of acarboxyl group-containing unsaturated monomer. The water-absorbing resinis not limited to the case in which the entire amount (100% by mass) isin the form of polymer and may contain other additives and the like tothe extent of retaining the above-mentioned characteristics. That is,even a water-absorbing resin composition is generally named as awater-absorbing resin in the present invention. The content of thepolyacrylic acid (salt)-type water-absorbing resin is preferably 70 to99.9 mass % relative to the entire water-absorbing resin, morepreferably 80 to 99.7 mass %, and still more preferably 90 to 99.5 mass%. The components other than the water-absorbing resin are preferablywater from the viewpoint of the water absorption speed and impactresistance of powder (particles) and may include, if necessary,additives described below.

(1-2) Polyacrylic Acid (Salt)-Type Water-Absorbing Resin

In the present invention, the “polyacrylic acid (salt)-typewater-absorbing resin” means a water-absorbing resin having a unitderived from acrylic acid and/or a salt thereof (hereinafter, referredto as an acrylic acid (salt)) as a main repeating unit. Concretely, itmeans a polymer inevitably containing an acrylic acid (salt) in anamount of 50 to 100 mol %, preferably 70 to 100 mol %, more preferably90 to 100 mol %, and particularly preferably substantially 100 mol % inthe total monomers (excluding a crosslinking agent) used forpolymerization. The salt as the polymer essentially contains awater-soluble salt, and preferably contains a monovalent salt, stillmore preferably an alkali metal salt or ammonium salt, particularly analkali metal salt, and further sodium salt.

(1-3) Initial Color Hue and Coloration with Time

In the present invention, the “initial color hue” means the color hue ofa water-absorbing resin immediately after production or immediatelyafter shipment to users, and in general, it is controlled based on thecolor hue before shipment from a plant. A method for measuring the colorhue may be methods (Lab value, YI value, WB value, etc.) described in WONo. 2009/005114.

The “coloration with time” means change of color hue of awater-absorbing resin caused during long time storage in an unused sateor caused during distribution. Since a water-absorbing resin is coloredwith time, the commodity value of paper diapers may be deteriorated. Thecoloration with time is caused during several months to several yearsand, therefore, it is verified by an accelerating test (acceleratingtest under high temperature and high humidity) disclosed in WO No.2009/005114.

(1-4) EDANA and ERT

The “EDANA” is an abbreviation of European Disposables and NonwovensAssociations. “ERT” is an abbreviation of measurement method (EDANARecommended Test Method) of a water-absorbing resin on the basis ofEuropean Standards (almost Global Standards). In this specification,unless otherwise specified, the physical properties of a water-absorbingresin or the like are measured based on ERT original text (PublishedLiterature: revised in 2002).

(a) “CRC” (ERT441.2-02)

The “CRC” is an abbreviation for Centrifuge Retention Capacity and meansabsorption capacity without load (simply sometimes referred to as“absorption capacity”). Specifically, the CRC is the absorption capacity(unit; g/g) after a water-absorbing resin is freely swollen in 0.9 mass% saline solution for 30 minutes and then is dehydrated in a centrifuge.

(b) “AAP” (ERT442.2-02)

The “AAP” is an abbreviation for Absorption Against Pressure and meansabsorption capacity under load. Specifically, the AAP is the absorptioncapacity (unit; g/g) in 0.9 mass % saline solution for 1 hour under aload of 2.06 kPa (0.3 psi, 21 [gf/cm²]). In the present invention, theAAP was measured under a loading condition of 2.06 kPa (0.3 psi, 21[gf/cm²]) or 4.83 kPa (0.7 psi, 50 [gf/cm²]).

(c) “Ext” (ERT 470.2-02)

The “Ext” is an abbreviation for Extractables and means the amount ofwater soluble components (amount of water-solubilized components).Specifically, measurement is carried out by adding 1 g of thewater-absorbing resin to 200 g of an 0.9 mass % aqueous saline solution,stirring the solution for 16 hours, and measuring the amount of adissolved polymer by pH titration (unit: mass %).

(d) “FSC” (ERT440.2-02)

The “FSC” is an abbreviation of Free Swell Capacity and means the ratioof free swelling. Concretely, it is the absorption capacity (unit;[g/g]) measured by immersing 0.20 g of a water-absorbing resin in anaqueous solution of 0.9 mass % of sodium chloride for 30 minutes andcarrying out the measurement without dehydration by a centrifuge.

(e) “Residual Monomers” (ERT410.2-02)

The “Residual Monomers” means the amount of monomers remaining in awater-absorbing resin. Specifically, the amount of monomers is a value(unit; ppm by mass) obtained by measuring, after 0.5 g of awater-absorbing resin is charged to 0.9 mass % saline solution and theresultant is stirred for 2 hours, the amount of monomers eluted in theaqueous solution by using high-pressure liquid chromatography.

(f) “PSD” (ERT420.2-02)

The “PSD” is an abbreviation for Particle Size Distribution and meansthe particle size distribution measured by sieving classification. Themass average particle diameter (D50) and the particle diameterdistribution width can be measured by the same method as in “(1) AverageParticle Diameter and Distribution of Particle Diameter” described inEuropean Patent No. 0349240, p. 7, lines 25-43.

(g) Other Physical Properties of Water-Absorbing Resin Defined in EDANA

“pH” (ERT400.2-02): The “pH” means pH of a water-absorbing resin.

“Moisture Content” (ERT 430.2-02): The moisture content means the watercontent percentage of a water-absorbing resin.

“Flow Rate” (ERT 450.2-02): The flow rate means the flow down speed of awater-absorbing resin.

“Density” (ERT 460.2-02): The density means the bulk specific density ofa water-absorbing resin.

“Respirable Particles” (ERT 480.2-02): The respirable particles mean therespirable dust of a water-absorbing resin.

“Dust” (ERT 490.2-02): The dust means the dust contained in awater-absorbing resin.

(1-5) Liquid Permeability

The “liquid permeability” means the flow of a liquid flowing amongparticles of swollen gel under a load or without a load. The “liquidpermeability” can be measured by SFC (saline flow conductivity) or GBP(Gel Bed Permeability) as a representative measurement method.

The “SFC (saline flow conductivity)” means the liquid permeability of0.9 g of a water-absorbing resin at a load of 0.3 psi for an aqueoussolution of 0.69 mass % of sodium chloride. It is measured according toan SFC testing method described in U.S. Pat. No. 5,669,894.

The “GBP” means the liquid permeability of a water-absorbing resin undera load or free expansion for 0.69 mass % physiological saline solution.It is measured according to a GBP testing method described in WO No.2005/016393 pamphlet.

(1-6) Heating State of an Apparatus

The “heating state of an apparatus” in the present invention means astate where an apparatus is heated with a heat source such as anelectric heater, steam, or hot air, but does not mean a state where theapparatus is kept warm with residual heat after the heat source isturned off. It includes a state where the heat source is turned off forcontrolling the temperature constant. The apparatus means an apparatusemployed in the respective steps and examples thereof include a dryer ina drying step and a heating treatment apparatus in a surface treatment(crosslinking) step.

(1-7) Interruption of Steps

In the present invention, the “Interruption of steps” means a statewhere a water-absorbing resin powder is substantially absent in anapparatus or not charged to or not discharged out of an apparatus incontinuous steps. That is, the “substantially absent state” means that awater-absorbing resin powder after surface treatment is taken out of theapparatus (heating treatment apparatus) (generally in an amount of notless than 95 mass % in the entire retention capability of the apparatus,preferably not less than 98 mass %, more preferably not less than 99mass %, and particularly preferably 100 mass %). A small amount of thewater-absorbing resin powder after surface treatment may be deposited,dropped, retained, or scattered in the apparatus. In addition, an idlingoperation of the apparatus is also included in the “interruption ofsteps” in the present invention. In the continuous steps, “a state notcharged to or not discharged out of an apparatus” means a state where awater-absorbing resin powder is stopped in an apparatus and a statewhere the apparatus is stopped.

(1-8) Continuous Production Such as Continuous Polymerization andContinuous Drying

In the present invention, “continuous production such as continuouspolymerization and continuous drying” means a state where thewater-absorbing resin is continuously charged to and continuouslydischarged out of an apparatus in the respective steps and an operationtime (duration) is preferably not shorter than 24 hours, more preferablynot shorter than 240 hours (10 days), and still more preferably notshorter than 720 hours (30 days). The present invention is applied suchcontinuous production (in respective steps of drying,surface-crosslinking, etc.).

(1-9) Others

In this description, “X to Y” showing a range means “not less than X andnot more than Y”. Additionally, the unit of mass “t (ton)” means “Metricton” and unless otherwise specified, “ppm” means “ppm by weight” or “ppmby mass”. Additionally, “ . . . acid (salt)” means “ . . . acid and/or asalt thereof” and “(meth)acrylic” means “acrylic and/or methacrylic”.

[2] PROCESS FOR PRODUCING WATER-ABSORBING RESIN

(2-1) Polymerization Step

This step is a step of obtaining a water-containing gel-like crosslinkedpolymer by polymerizing an aqueous solution containing acrylic acidand/or a salt thereof (hereinafter, referred to as an “acrylic acid(salt)” as a main component.

(a) Monomer (Excluding Crosslinking Agent)

A water-absorbing resin to be obtained by the present invention isproduced by using, as a raw material (monomer), an aqueous solutioncontaining an acrylic acid (salt) as a main component and generallypolymerized in an aqueous solution state. The monomer concentration(solid content concentration) of the aqueous monomer solution isgenerally 10 to 90 mass % and preferably 20 to 80 mass %. Polymerizationwith a high monomer concentration (not lower than 35 mass %, still morepreferably not lower than 40 mass %, particularly preferably not lowerthan 45 mass %, and the saturated concentration as the upper limit,still more preferably not higher than 80 mass % and particularlypreferably not higher than 70 mass %) is one of most preferableexamples.

In the case a monomer is polymerized in an aqueous solution state, ifnecessary, surfactants, polymer compounds such as a polyacrylic acid(salt), starch, cellulose, and polyvinyl alcohol, various kinds ofchelating agents, and various kinds of additives may be added in anamount of 0 to 30 mass % relative to the monomer.

The hydrogel obtained by the polymerization of the aqueous solution,preferably has at least partially neutralized acid groups in the polymerfrom the viewpoint of water absorption properties. The neutralizationmay be carried out before polymerization (monomer), duringpolymerization, or after polymerization (hydrogel) of acrylic acid andfrom the viewpoint of improvement of the productivity of thewater-absorbing resin, AAP (absorption capacity under load), SFC (salineflow conductivity), and the like, it is preferable to carry outneutralization before polymerization of acrylic acid. That is, it ispreferable to use neutralized acrylic acid (that is, a partiallyneutralized salt of acrylic acid) as a monomer.

The neutralization ratio of the neutralization is not particularlylimited and it is preferably 10 to 100 mol %, more preferably 30 to 95mol %, still more preferably 50 to 90 mol %, and particularly morepreferably 60 to 80 mol % of the acid group. In the case theneutralization ratio is less than 10 mol %, particularly CRC (absorptioncapacity without load) is sometimes considerably lowered and, therefore,it is not preferable.

In the case an acrylic acid (salt) is used as a main component in thepresent invention, hydrophilic or hydrophobic unsaturated monomers(hereinafter, sometimes also referred to as “other monomers”) may beused besides an acrylic acid (salt). The other monomers is notparticularly limited and may include methacrylic acid, maleic acid,maleic anhydride, 2-(meth)acrylamido-2-methylpropanesulfonic acid,(meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrrolidone,N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl (meth)acrylate,methoxypolyethylene glycol (meth)acrylate, polyethylene glycol(meth)acrylate, stearyl acrylate, their salts, etc. In the case theseother monomers are used, their use amount is not particularly limited aslong as the water absorption property of the water-absorbing resin to beobtained is not deteriorated and it is preferably not more than 50 mass% and more preferably not more than 20 mass % relative to the totalweight of the monomers.

(b) Salt for Neutralization

A basic substance to be used for neutralizing acrylic acid as a monomeror a polymer (hydrogel) after the polymerization is not particularlylimited and is preferably monovalent basic substances, for example,alkali metal hydroxides such as sodium hydroxide, potassium hydroxide,lithium hydroxide, (hydrogen) carbonate salts such as sodium (hydrogen)carbonate, potassium (hydrogen) carbonate, and the like, and isparticularly preferably sodium hydroxide. The temperature at the time ofneutralization (neutralization temperature) is also not particularlylimited and preferably 10 to 100° C. and more preferably 30 to 90° C.Regarding the neutralization treatment conditions and the like otherthan the above-mentioned ones, the conditions and the like disclosed inWO No. 2006/522181 and U.S. Pat. No. 6,388,000 are preferably employedin the present invention.

(c) Crosslinking Agent (Internal Crosslinking Agent)

In the present invention, from the viewpoint of water absorptionproperties of the water-absorbing resin to be obtained, it isparticularly preferable to use a crosslinking agent (hereinafter,sometimes referred to as an “internal crosslinking agent”). An internalcrosslinking agent to be used may be compounds having 2 or morepolymerizable double bonds in one molecule and polyfunctional compoundshaving 2 or more functional groups capable of forming covalent bonds byreaction with a carboxyl group in one molecule. For examples, one ormore kinds of polymerizable crosslinking agents with the acrylic acid,reactive crosslinking agents with a carboxyl group, and crosslinkingagents having both of these properties can be exemplified. Concreteexamples are, as a polymerizable crosslinking agent, compounds having atleast two polymerizable double bonds in a molecule such asN,N′-methylene bisacrylamide, (poly)ethylene glycol di(meth)acrylate,(polyoxyethylene)trimethylolpropane tri(meth)acrylate, andpoly(meth)allyloxyalkanes. Further, examples of the reactivecrosslinking agent are covalent-binding crosslinking agents, for examplepolyglycidyl ether such as ethylene glycol diglycidyl ether, polyalcohols such as propanediol, glycerin, sorbitol and the like, andion-binding crosslinking agents such as polyvalent metal compounds ofaluminum salt and the like. Among these crosslinking agents, from theviewpoint of water absorbent properties, polymerizable crosslinkingagents with the acrylic acid, particularly, acrylate type, allyl type,and acrylamide type polymerizable crosslinking agents are preferablyused. These internal crosslinking agents may be used alone or two ormore kinds of them may be used in combination. The use amount of theinternal crosslinking agent is preferably 0.001 to 5 mol %, morepreferably 0.005 to 2 mol %, still more preferably 0.01 to 1 mol %, andparticularly preferably 0.03 to 0.5 mol % relative to the monomerexcluding the crosslinking agent from the viewpoint of physicalproperty.

(d) Other Minor Components

In the present invention, from the viewpoint of color hue stability andresidual monomers, the content of protoanemonin and/or furfural inacrylic acid is preferably 0 to 10 ppm, more preferably 0 to 5 ppm, andstill more preferably 0 to 1 ppm. For the same reason, the content of analdehyde component other than furfural and/or maleic acid in acrylicacid is preferably 0 to 5 ppm, more preferably 0 to 3 ppm, still morepreferably 0 to 1 ppm, and particularly preferably 0 ppm (underdetection limit). Examples of the aldehyde component other than furfuralinclude benzaldehyde, acrolein, acetaldehyde and the like. In order toreduce the residual monomers, the content of acrylic acid dimer ispreferably 0 to 500 ppm, more preferably 0 to 200 ppm, and still morepreferably 0 to 100 ppm.

In the present invention, from the viewpoint of polymerizationstability, it is preferable to include methoxyphenols in the unsaturatedmonomers and it is more preferable to include p-methoxyphenol. Thecontent of methoxyphenols is preferably 1 to 250 ppm, more preferably 5to 200 ppm, still more preferably 10 to 160 ppm, and particularlypreferably 20 to 100 ppm relative to the monomer (acrylic acid).

(e) Other Components in Aqueous Monomer Solution

In order to improve various physical properties of the water-absorbingresin to be obtained by the present invention, the following substancesmay be added as arbitrary components to the aqueous monomer solution.That is, a water-soluble resin or a water-absorbing resin such asstarch, a polyacrylic acid (salt), polyvinyl alcohol, or polyethyleneimine may be added in an amount of, for example, 0 to 50 mass %,preferably 0 to 20 mass %, more preferably 0 to 10 mass %, and stillmore preferably 0 to 3 mass % relative to the monomer. Further,additives such as various kinds of foaming agents (carbonates, azocompounds, air bubbles, etc.), surfactants, various kinds of chelatingagents, hydroxycarboxylic acids, and reducing inorganic salts may beadded in an amount of, for example, 0 to 5 mass % and preferably 0 to 1mass % relative to the monomer.

Among them, in the case of aiming at suppression of coloration with thetime (improvement of color hue stability at the time of long timestorage under high temperature and high humidity) and improvement ofurine resistance (gel deterioration prevention) of the water-absorbingresin, a chelating agent, a hydroxycarboxylic acid, and a reducinginorganic salt are preferably used and a chelating agent is particularlypreferably used. The use amount in this case is preferably 10 to 5000ppm, more preferably 10 to 1000 ppm, still more preferably 50 to 1000ppm, and particularly preferably 100 to 1000 ppm relative to thewater-absorbing resin. The compounds disclosed in WO No. 2009/005114 andEU Patent Application Publication Nos. 2057228 and 1848758 may be usedfor the chelating agent, hydroxycarboxylic acid, and reducing inorganicsalt.

(f) Polymerization Initiator

A polymerization initiator to be used for the present invention is notparticularly limited and can be selected properly in accordance with thepolymerization mode. Examples of the polymerization initiator mayinclude a heat decomposition type polymerization initiator, aphotodecomposition type polymerization initiator, a redox typepolymerization initiator and the like. Examples of the heatdecomposition type polymerization initiator may include persulfate suchas sodium persulfate, potassium persulfate, and ammonium persulfate;peroxides such as hydrogen peroxide, tert-butyl peroxide, and methylethyl ketone peroxide; azo compounds such as2,2′-azobis(2-amindinopropane) dihydrochloride,2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, and the like.Examples of the photodecomposition type polymerization initiator mayinclude benzoin derivatives, benzyl derivatives, acetophenonederivatives, benzophenone derivatives, azo compounds, and the like.Examples of the redox type polymerization initiator may include theabove-mentioned persulfate or peroxides in combination with reducingcompounds such as L-ascorbic acid and sodium hydrogen sulfite. Further,combination use of a heat decomposition type polymerization initiatorand a photodecomposition type polymerization initiator can also beexemplified as a preferable embodiment. The use amount of thosepolymerization initiators is preferably 0.0001 to 1 mol % and morepreferably 0.001 to 0.5 mol % relative to the monomer. In the case theuse amount of a polymerization initiator exceeds 1 mol %, it may causecoloration of the water-absorbing resin and, therefore, it is notpreferable. On the other hand, if the use amount of a polymerizationinitiator is below 0.0001 mol %, it may possibly increase the residualmonomer amount and, therefore, it is not preferable.

In place of use of the polymerization initiator, polymerization may becarried out by irradiation with an active energy beam such as radiationbeam, electron beam, or ultraviolet rays and also polymerization may becarried out by employing the active energy beam and a polymerizationinitiator in combination.

(g) Polymerization Method (Crosslinking Polymerization Step)

In the present invention, from the viewpoint of water absorptionproperties of a water-absorbing resin to be obtained, ease ofpolymerization control in the case of polymerization of the aqueousmonomer solution, and the like, generally, aqueous solutionpolymerization or reverse phase suspension polymerization is employed,and aqueous solution polymerization is preferable and continuous aqueoussolution polymerization is more preferably employed. Among them,continuous aqueous solution polymerization is preferably employed forproduction of a water-absorbing resin in a huge scale with a largeproduction amount for one line. The production amount is 0.5 [t/hr] orhigher, more preferably 1 [t/hr] or higher, still more preferably 5[t/hr] or higher, and particularly preferably 10 [t/hr] or higher.

Examples of a preferable embodiment of the continuous aqueous solutionpolymerization include continuous belt polymerization (U.S. Pat. Nos.4,893,999 and 6,241,928, US Patent Application Publication No.2005/215734, etc.) and continuous kneader polymerization (U.S. Pat. Nos.6,987,151, 670141, etc.).

One of most preferable examples of the continuous aqueous solutionpolymerization is high temperature starting polymerization at apolymerization starting temperature of not lower than 30° C., preferablynot lower than 35° C., more preferably not lower than 40° C., still morepreferably not lower than 50° C., and particularly preferably not lowerthan 60° C. (upper limit is boiling point) or high monomer concentrationpolymerization with a monomer concentration of not lower than 35 mass %,preferably not lower than 40 mass %, and particularly preferably notlower than 45 mass % (upper limit is the saturated concentration). Thepolymerization starting temperature is defined in accordance with theliquid temperature of an aqueous monomer solution immediately before thesolution is supplied to a polymerization apparatus, the conditions, andthe like disclosed in U.S. Pat. Nos. 6,906,159, 7,091,253, etc. can beemployed preferably for the present invention.

From the viewpoint of improvement of physical property of thewater-absorbing resin to be obtained and drying efficiency, it ispreferable to evaporate water at the time of polymerization and obtain awater-absorbing resin with a higher solid content concentration. Theincrease degree of the solid content from the aqueous monomer solution(solid content of hydrogel after polymerization—solid content of monomerbefore polymerization) is preferably not less than 1 mass %, morepreferably 2 to 40 mass %, and still more preferably 3 to 30 mass %. Theincrease degree is preferably within a range of giving awater-containing gel-like crosslinked polymer containing not more than80 mass % of solid content.

The polymerization can be carried out in atmospheric air; however, fromthe viewpoint of prevention of coloration, it is preferable to carry outthe polymerization in an inert gas atmosphere of nitrogen, argon, andthe like (e.g., with not more than 1% by volume of oxygenconcentration). It is also preferable to carry out the polymerizationafter replacing the dissolved oxygen in a monomer or a solutioncontaining a monomer with an inert gas (e.g., dissolved oxygenconcentration; less than 1 mg/L). The polymerization can be carried outunder any pressure; that is, under reduced pressure, under normalpressure, or under pressurization.

(2-2) Finely Crushing Step of Water-Containing Gel-Like CrosslinkedPolymer (Gel-Crushing Step)

This step is a step of finely crushing a water-containing gel-likecrosslinked polymer obtained in the polymerization step to obtain aparticulated water-containing gel-like crosslinked polymer (hereinafter,referred to as a “particulated hydrogel”).

The hydrogel obtained in the polymerization step may be dried as it is,but in order to solve the above-mentioned problems, the hydrogel may begel-crushed, if necessary, preferably during the polymerization or afterthe polymerization by a crusher (a kneader, a meat chopper, a cuttermill, etc.) to be particulate. That is, between the polymerization stepby the continuous belt polymerization or the continuous kneaderpolymerization and the drying step, a step of finely crushing of thehydrogel (hereinafter, also referred to as “gel-crushing”) may be addedfurther. In addition, even in the case the gel is finely crushed bydispersion in a solvent at the time of polymerization such asreverse-phase suspension polymerization, the finely crushing step(finely crushing during polymerization) is included in the finelycrushing step of the present invention, and the finely crushing ispreferably carried out using a crusher.

The temperature of the hydrogel at the time of gel-crushing is kept ator increased to preferably 40 to 95° C. and more preferably 50 to 80° C.from the viewpoint of physical properties. The mass average particlediameter (D50) of the particulated hydrogel after the gel-crushing ispreferably 0.5 to 4 mm, more preferably 0.3 to 3 mm, and still morepreferably 0.5 to 2 mm. If the mass average particle diameter (D50) ofthe particulated hydrogel is within the above-mentioned range, drying isefficiently carried out and, therefore, it is preferable. The ratio ofthe particulated hydrogel having a particle diameter of not smaller than5 mm is preferably 0 to 10 mass % and more preferably 0 to 5 mass % inthe entire particulated hydrogel. Herein, the particle diameter of theparticulated hydrogel is measured by a wet classification methoddescribed in paragraph [0091] in JP-A-2000-63527.

(2-3) Drying Step

The drying step is a step of drying a particulated water-containinggel-like crosslinked polymer obtained in finely crushing step during thepolymerization or after the polymerization.

Various kinds of dryers can be used as the dryer to be used in thedrying step in the present invention without any particular limitation.An air ventilating band type continuous dryer and a fluidized bed dryerare preferably used and an air ventilating band type continuous dryer ismore preferably used. The air ventilating band type continuous dryer canefficiently carry out drying.

As for the operation time of the drying, continuous drying is carriedout preferably for not shorter than 24 hours, more preferably for notshorter than 120 hours, still more preferably for not shorter than 240hours, and particularly preferably for not shorter than 720 hours.

(a) Dryer

A dryer to be used in the present invention is preferably an airventilating belt type dryer (belt type dryer) and also, if necessary,one or more kinds of a heat conduction and heat transfer type dryer, aradiation heat transfer type dryer, a hot air heat transfer type dryer,an induction heating type dryer, etc. may be used and from the viewpointof quick drying, a hot air heat transfer type dryer (hereinafter,referred to as a hot air dryer) is preferable. Examples of the hot airdryer include an air ventilating belt (band) type, air ventilatingcircuit type, air ventilating vertical type, parallel flow belt (band)type, air ventilating tunnel type, air ventilating groove stirring type,fluidized bed type, current type, spraying type and the like dryers andin the present invention, from the viewpoint of the physical propertycontrol, an air ventilating belt type dryer is preferable. It ispossible to use another dryer in combination, but it is preferable tocarry out drying by solely the air ventilating belt type dryer (belttype dryer).

Examples of the heat source of the dryer include an electric heater, andvarious kinds of heated gases such as steam and hot air. Heated steam(not lower than 100° C.) is particularly preferably used as a heatsource. The heated steam is recycled and reheated through a heatexchanger if necessary, and used in the drying step of thewater-absorbing resin or in another step. The heating temperature may beconstant or fluctuated.

The drying temperature is in a temperature range (hot air temperature)of generally 100 to 250° C., preferably 100 to 220° C., more preferably120 to 200° C., and particularly preferably 150 to 190° C. It is moreefficient that the velocity of the hot air passing in a drying chamberis higher in a range of scarcely blowing out the polymer and thevelocity is preferably 0.1 to 5 [m/s] and more preferably 0.5 to 3[m/s]. If the velocity is below 0.1 [m/s], it takes such a long time todry out to a prescribed moisture content that a huge dryer is required.On the other hand, if the velocity exceeds 5 [m/s], the amount of thepolymer flying out of the drying chamber is increased and stableoperation becomes difficult. The drying time (the time from charging ofthe particluated water-containing gel-like crosslinked polymer to thedryer to discharge of the same particluated water-containing gel-likecrosslinked polymer in form of a dried material out of the dryer)depends on the surface area and moisture content of the polymer, thetype of the dryer, and the air blow quantity and is selected to give anintended moisture content. For example, the drying time is properlyselected in a range of preferably 1 minute to 1 hour.

The present invention is applied for huge scale continuous drying with abelt length of the dryer in a range of 5 to 100 m, further 10 to 70 m,and particularly 20 to 60 m. The width of the belt is not particularlylimited and it is properly determined in a range of generally 0.5 to 10m and further 1 to 5 m. The ratio between the width direction and thelongitudinal direction may be determined in accordance with the purpose,but it is preferably longer in the longitudinal direction (proceedingdirection) than in the width direction and the ratio is properlydetermined in a range of generally 3 to 500 times and further 5 to 100times.

The drying in the present invention is preferably carried out on acontinuous air ventilating belt and examples of the air ventilating beltinclude a metal net (e.g., meshes of 1000 to 45 μm) and a punching metaland a punching metal is preferably used. The shape of holes of apunching metal may be various and examples thereof include round holes,elliptic holes, rectangular holes, hexagonal holes, long round holes,long rectangular holes, diamond holes, cross holes, and a plurality ofthese holes in combination and the arrangement of the holes may be inzigzag or in parallel arrangement. The belt may have holes formedthree-dimensionally like louvers but preferably have holes in a flatstructure. The pitch direction of the belt may be vertical, transverse,or slanting to the belt proceeding direction and may be a combination ofthese. The size of the holes and hole ratio of the punching metal willbe described later.

The conveying speed of the particulated water-containing gel-likecrosslinked polymer on the air ventilating belt may be properly adjustedin accordance with the belt width, the belt length, the productionamount, and the drying time and from the viewpoint of the load to thebelt driving apparatus, durability and the like, it is preferably 0.3 to5 [m/min], more preferably 0.5 to 2.5 [m/min], still more preferably 0.5to 2 [m/min], and particularly preferably 0.7 to 1.5 [m/min].

In order to achieve the present invention, it is preferable to changethe temperature, dew point, and air blow quantity in multi-stages and,therefore, the air ventilating belt type dryer preferably has not lessthan 5 chambers, particularly not less than 6 chambers, and further notless than 8 chambers. The upper limit may be properly set in accordancewith the scale or the like and generally about 20 chambers suffice.

(b) Surface Occupancy

In the production process of the present invention, in the case an airventilating band dryer is used, the surface occupancy on the belt isgenerally 85 to 100%, preferably 87 to 100%, more preferably 87 to 99%,particularly preferably 90 to 98%, and most preferably 93 to 97%. Thesurface occupancy is defined as the area ratio (percentage) of thelayered material of the particulated hydrogel occupying the airventilating belt face to the air ventilating belt area (A) in theinitial period of the drying step. The air ventilating belt area (A)includes the areas of the holes. The area (B) of the air ventilatingbelt face on which the layered material of the particulated hydrogel inthe initial drying period occupies is defined by the occupying area ofthe layered material of the particulated hydrogel in the section. Thesurface occupancy (B/A×100(%)) is defined by the air ventilating beltarea (A) that is defined aforementioned and the area (B) occupied by thelayered material of the particulated hydrogel. If the surface occupancyexceeds 99% or less than 85%, it is found that the physical propertiesof a water-absorbing resin tend to be deteriorated and the dropping andscattering ratio, drying efficiency, and continuous drying property tendto be lowered. Unoccupied places on the belt may be properly determinedand parts where no hydrogel is layered may be formed in certain setpositions in the center part, both end parts, and middle parts andpreferably a prescribed region where no hydrogel is set is formed inboth end parts.

(c) Hole Ratio and Holes

In the production process of the present invention, in the case an airventilating band dryer is used, the hole ratio of the punching metal ispreferably 15 to 50%, more preferably 20 to 45%, and particularlypreferably 25 to 40%. The hole ratio is determined in accordance withthe holes, pitches (P) and the like, and in the case there is no hole ina certain prescribed region, for example, the punching metal has rims,the hole ratio is defined based on the area including the region. If thehole ratio is out of the above-mentioned range, it is found that thephysical properties of a water-absorbing resin tend to be deterioratedand the drying efficiency and continuous drying property tend to belowered.

The area of one hole (in the case where there are a plurality of typesof holes, the area is defined as an average opening area) is preferablylarger than the cross-sectional area of one particle of the particulatedhydrogel, more preferably in a range of 2 to 100 times as large, andstill more preferably in a range of 4 to 50 times as large. The maximumopening distance of each hole (e.g., the diameter in the case of acircle, or the longer diameter in the case of an ellipse) is preferablylarger than the mass average particle diameter of the particulatedhydrogel, more preferably in a range of 2 to 100 times as large, andstill more preferably in a range of 4 to 50 times as large. The averageopening area of the holes is 5 to 500 mm², preferably 10 to 100 mm², andparticularly preferably 15 to 50 mm². In the case the area is smallerthan the above-mentioned range, the drying efficiency is lowered and inthe case it is larger, the yield of the dried material is lowered and,therefore, these cases are not preferable.

(d) Resin Solid Content

As described above, in the polymerization step in the production processof the present invention, continuous kneader polymerization orcontinuous belt polymerization, which is carried out while evaporatingwater, is preferable. In this polymerization step, the increase degreeof the solid content (the difference between the solid content of theaqueous monomer solution and the gel solid content) is not less than 1mass %, further not less than 2 mass %, and particularly not less than 5mass % and owing to the solid content increase, the water-absorbingresin with high physical properties can be obtained with highproductivity and low energy. In the drying step, similarly, increase ofthe resin solid content not only reduces energy but also causes apreferable effect on decrease of adhesion to the dryer.

From the viewpoint of solid content increase in the drying step,prevention of adhesion of the hydrogel to the dryer, particle sizecontrol of a product and the like, the water-absorbing resin fineparticles obtained in the classification step can be recycled to theparticulated water-containing gel-like crosslinked polymer beforedrying. The solid content of the particulated water-containing gel-likecrosslinked polymer before introduction into the drying step ispreferably not less than 45 mass %. The particulated water-containinggel-like crosslinked polymer is dried in a scale of 1 [t/hr] or more.The resin solid content can be controlled based on the monomerconcentration, water evaporation at the time of polymerization,recycling of the fine powder, and the like, and increase of the resinsolid content can not only reduce the energy but also adhesion to thedryer.

(2-4) Pulverizing Step and Classification Step (Particle Size Adjustmentafter Drying)

These steps are steps for pulverizing and classifying the dried materialobtained in the drying step to obtain a water-absorbing resin powder.The water-absorbing resin powder is a water-absorbing resin before thewater-absorbing resin is subjected to the following surface-crosslinkingtreatment. The dried material obtained in the drying step can be used asa dried powder as it is, but the particulated hydrogel may possibly beagglomerated at the time of drying to form block-like agglomerates. Thisphenomenon is observed particularly in a band dryer and pulverizing orrough crushing (break into small pieces) is needed. Further, in order toimprove the physical properties in the surface-crosslinking stepdescribed later, it is preferable to control the water-absorbing resinpowder to have a specified particle size. The particle size may beproperly controlled not only in the pulverizing step and classificationstep but also in the polymerization step (particularly, reverse-phasesuspension polymerization), the fine powder recovery step, thegranulation step, etc. Hereinafter, the particle size is defined by astandard sieve (JIS Z8801-1 (2000)).

A pulverizer to be used in the pulverizing step is not particularlylimited and conventionally known pulverizers may be used. Specificexamples thereof include a roll mill, a hammer mill, a roll granulator,a jaw crusher, a gyrectory crusher, a cone crusher, a roll crusher, acutter mill, etc. Among them, a multi-stage roll mill or a rollgranulator is preferably used from the viewpoint of particle sizecontrol. In the classification step, various kinds of classificationapparatuses such as sieving classification and air currentclassification can be used.

The classification step in the present invention is carried outinevitably before surface-crosslinking and/or aftersurface-crosslinking, preferably before surface-crosslinking, and stillmore preferably carried out twice in total before surface-crosslinkingand after surface-crosslinking.

From the viewpoint of improvement of physical properties of awater-absorbing resin to be obtained in this step, it is preferable tocontrol the particle size to be in the following range. That is, themass average particle diameter (D50) of the water-absorbing resin powder(before surface-crosslinking) is preferably 200 to 600 more preferably200 to 550 μm, still more preferably 250 to 500 μm, and particularlypreferably 350 to 450 μm. The ratio of fine particles which pass througha sieve with meshes of 150 μm (JIS standard sieve) is preferably 0 to 5mass %, more preferably 0 to 3 mass %, and still more preferably 0 to 1mass % relative to the entire water-absorbing resin powder. The ratio ofcoarse particles which do not pass through a sieve with meshes of 850 μm(JIS standard sieve) is preferably 0 to 5 mass %, more preferably 0 to 3mass %, and still more preferably 0 to 1 mass % relative to the entirewater-absorbing resin powder. The particle size is measured by a methoddisclosed in WO No. 2004/69915 and EDANA-ERT 420.2.-02 (Particle SizeDistribution).

(2-5) Surface Treatment Step

The process for producing a water-absorbing resin of the presentinvention is a process for producing a water-absorbing resin involving apolymerization step of polymerizing an aqueous unsaturated monomersolution, a drying step of drying a particulated water-containinggel-like crosslinked polymer obtained in a finely crushing step duringthe polymerization or after the polymerization, a pulverizing step afterdrying, a classification step after drying, a surface treatment step fora water-absorbing resin powder after the classification step, and, ifnecessary, a second classification step after the surface treatment step(e.g., see FIG. 1) and is characterized in that the surface treatmentstep is interrupted with a heating treatment apparatus kept in theheated state and thereafter the surface treatment step is restarted. The“interruption of surface treatment step” means a state where thewater-absorbing resin powder is substantially absent in a heatingtreatment apparatus, or not charged to or not discharged out of aheating treatment apparatus in continuous surface treatment step.

As described above, the process for producing the water-absorbing resininvolves a large number of continuously carried out steps such as apolymerization step, a finely crushing step, a drying step, apulverizing step, a classification step, and a surface treatment step.At the time of periodic maintenance and a temporal trouble (an operationtrouble in some steps), it is necessary to stop all of the steps and tostop the continuous production flow of the water-absorbing resin. Theoperation is restarted after these countermeasures are taken, and insuch a case, because the quality of the water-absorbing resin to beobtained is not stabilized or excess energy is needed to restart theoperation, apparatuses sometimes get out of order and the operation maybe stopped again. Further, coloring foreign matters (mainly a discoloredmatter of the water-absorbing resin) are sometimes mixed at the time ofrestarting of the operation.

In the present invention, among above-mentioned steps, assuming that aheating treatment apparatus used for the surface treatment step (heatingtreatment step) is stopped, the inventors have paid attention to amethod of stopping the heating treatment apparatus by which theabove-mentioned problem can be solved. That is, in the presentinvention, the inventors have found that the above-mentioned problem canbe solved by keeping the heating treatment apparatus in a heated stateeven in a interruption period of the heating treatment step. The heatingtreatment apparatus is smoothly operated at the time of restarting ofthe operation, no colored foreign matter is mixed in the water-absorbingresin, and a continuous surface treatment can be stably carried out bycarrying out the above-mentioned operation. “Keeping the heatingtreatment apparatus in a heated state” means consecutive supply of aheat source and a heated state based only on remaining heat withoutsupply of a heat source during the stop of the heating treatment step isnot included in the heated state.

Examples of the heat source include an electric heater, and variouskinds of heated gases such as steam and hot air, and consecutive supplyof heated gas is preferable. Heated steam (not lower than 100° C.) isparticularly preferably used as a heat source. The heated steam isrecycled and reheated through a heat exchanger if necessary, and used inthe drying step of the water-absorbing resin or in another step. Theconsecutive supply of a heat source to the dryer may be carried out soas to give a desired heating temperature or keep a temperature range asdescribed below and it may be continuous heating or intermittent heating(On/Off). The heating temperature may be constant or fluctuated.

The heating treatment apparatus and the water-absorbing resin powderremaining in the heating treatment apparatus are heated preferably by amethod of blowing air, heated by the heat source, into the heatingtreatment apparatus or by a method of warming the heating treatmentapparatus by tracing.

Herein, the heating treatment step may be carried out in any of acontinuous manner, a semi-continuous manner, and a batch manner in thepresent invention and especially, a continuous heating treatment step ispreferably employed. That is, at the time of stopping the continuousheating treatment step, the heating treatment apparatus is stopped whilebeing kept in a heated state and thereafter the heating treatment isrestarted. If heating treatment is restarted after the heating treatmentapparatus is stopped completely, excess energy (torque) is needed forpaddle at the time of starting the operation, or a colored foreignmatter may be mixed.

As for the treatment time of the heating treatment, continuous heatingtreatment is carried out preferably for not shorter than 24 hours, morepreferably for not shorter than 120 hours, still more preferably for notshorter than 240 hours, and particularly preferably for not shorter than720 hours. The time from stopping of heating treatment to starting ofheating treatment (the interruption period of the heating treatment stepin the present invention) is preferably not shorter than 0.5 hours, morepreferably not shorter than 0.5 hours and within 100 days, still morepreferably not shorter than 1 hour and within 50 days, particularlypreferably not shorter than 5 hours and within 20 days, and mostpreferably not shorter than 10 hours and within 15 days. In the case theinterruption period of the heating treatment step is shorter than 0.5hours, since the amount of temperature decrease from the heatingtreatment temperature before stopping is small, the effect of stoppingin the heated state is small. On the other hand, in the case theinterruption period of the heating treatment step exceeds 100 days,since the heating treatment apparatus is not used for a long period, theenergy use continues for a long period and it results in disadvantage ofthe energy cost.

The temperature of the heating treatment apparatus (atmospheretemperature in the heating treatment apparatus) during the interruptionperiod of the heating treatment step is not particularly limited as longas the heated state is consecuted and from the viewpoint of the energyand coloration issue, the temperature is preferably lower than theheating treatment temperature. Concretely, it is lower than the heatingtreatment temperature preferably by not less than 10° C., and morepreferably by not less than 20° C., not less than 30° C., and not lessthan 40° C. in this order. The lower limit of temperature of the heatingtreatment apparatus during the interruption of the heating treatmentstep is preferably not lower than 40° C., more preferably not lower than50° C., and still more preferably not lower than 60° C. If thetemperature of the heating treatment apparatus is lower than 40° C.,excess energy is needed at the time of starting the paddle and a coloredforeign matter may be mixed and, therefore, it is undesirable.Consequently, the temperature of the heating treatment apparatus duringthe interruption period of the heating treatment step is preferably notlower than 40° C., more preferably not lower than 60° C., and still morepreferably 80 to 140° C. In addition, in the case the water-absorbingresin remains in the heating treatment apparatus such as a case where atrouble occurs downstream of the heating treatment apparatus, if theheating treatment apparatus is stopped at the heating treatmenttemperature as it is during the interruption period, the absorptioncapacity of the water-absorbing resin powder remaining in the heatingtreatment apparatus is lowered. Therefore, at the time of restarting ofthe heating treatment apparatus, it may take a long time to obtain awater-absorbing resin with a desired absorption capacity. On the otherhand, if the heating treatment apparatus is stopped at lower than 40°C., other than the above-mentioned problems, there occurs anotherproblem that the physical property of the water-absorbing resin isfluctuated by lots at the time of restarting of the heating treatmentapparatus and in many cases the water-absorbing resin often becomesinferior in the absorption capacity and liquid permeability. The reasonfor this is assumed as follows: because the water-absorbing resin powderin the inside of the heating treatment apparatus is agglomerated andremains in the inside of the heating treatment apparatus without beingdischarged even after restart of the heating treatment apparatus, auniform heating treatment is continuously inhibited.

It is preferable that retained substance (or its agglomerated particles)of the water-absorbing resin powder is removed from the heatingtreatment apparatus during the interruption period of the heatingtreatment step. Removal of the retained substance (including the droppedretained substance) gives the water-absorbing resin free from a coloredforeign matter (e.g., the water-absorbing resin burned out to be yellow,brown, dark brown, or black). The colored foreign matter is produced bylong time retention of the retained substance of the water-absorbingresin powder in the heating treatment apparatus. Consequently, in thecase of continuous operation for not shorter than 5 days, not shorterthan 10 days, or furthermore not shorter than 20 days, it is preferableto periodically remove and clean up the retained substance of thewater-absorbing resin powder. The form of the retained substance is notparticularly limited, but it is generally particulate, and its particlesare retained in the heating treatment apparatus by dropping from thepaddle of the heating treatment apparatus, scattering, or adhering tothe heating treatment apparatus. A method for removing the retainedsubstance may be vacuum suction, removal by a brush, or blowing up with(pressurized) air. At the time of removal of (particles of) the retainedsubstance from the heating treatment apparatus, the heating treatmentapparatus may be in the heated state; however, in the case a workercleans the inside of the heating treatment apparatus, from the viewpointof the ease of the removal work, it is preferable to quickly carry outthe cleaning work after the temperature is decreased to around roomtemperature. That is, cleaning may be started within 100 hours. Cleaningis started preferably within 48 hours, more preferably within 24 hours,still more preferably within 12 hours, and particularly preferablywithin 6 hours. If times passes while the heating is stopped, adhesionof the retained substance becomes intense as described above and thecleaning may become difficult.

In the present invention, a step for reforming the periphery of particlesurface for the water-absorbing resin powder (particles) with theprescribed particle size is called as a surface treatment step. Herein,the “surface treatment” may be surface-crosslinking and also addition ofvarious kinds of additives and polymers to the particle surface andpreferably, surface-crosslinking by a thermal reaction is indispensablycarried out. The surface treatment other than surface-crosslinking maybe addition of a water-soluble or water-insoluble polymer, a lubricant,a chelating agent, a deodorant, an antibacterial agent, water, asurfactant, water-insoluble fine particles, an antioxidant, a reducingagent, etc. These agents may be added and mixed in an amount ofpreferably 0 to 30 mass % and more preferably 0.01 to 10 mass % to thewater-absorbing resin powder or water-absorbing resin particles aftersurface-crosslinking. These agents may be mixed and heated up to theabove-mentioned upper limit in place of the followingsurface-crosslinking agent. Hereinafter, a surface-crosslinking step isexplained as a representative and the following respective steps ofmixing, heating, and stopping can be properly be applied for the surfacetreatment step, which is a superordinate concept.

(a) Mixing Step (Surface-Crosslinking Step)

In the present invention, in order to improve the water absorptionperformance, preferably a surface-crosslinking step is further involved.In the present invention, coloration is scarcely caused and a whiterwater-absorbing resin can be obtained by the surface-crosslinkingtreatment. The following respective steps can be applied preferably forsurface-crosslinking of a water-absorbing resin, particularly for hightemperature surface-crosslinking.

The surface-crosslinking step in the present invention is composed of astep of mixing a water-absorbing resin powder with asurface-crosslinking agent, a step of heating treatment for the mixture,and a cooling step carried out if necessary.

(Covalent Bonding Surface-Crosslinking Agent)

A surface-crosslinking agent to be used for the present invention is notparticularly limited and various kinds of organic and inorganicsurface-crosslinking agents can be mentioned. Among them, an organicsurface-crosslinking agent alone or combination use of an organicsurface-crosslinking agent and an ion bonding surface-crosslinking agentis preferable. A covalent bonding surface-crosslinking agent ispreferably used as the organic surface-crosslinking agent, preferableexamples to be used are dehydration reactive surface-crosslinking agentssuch as polyhydric alcohol compounds, epoxy compounds, polyaminecompounds and their condensation products with haloepoxy compounds,oxazoline compounds (mono-, di-, or poly-)oxazolidinone compounds,alkylene carbonate compounds. Particularly dehydration reactivesurface-crosslinking agents such as a polyhydric alcohol compound, analkylene carbonate compound, and an oxazolidinone compound, whichrequire high temperature reaction are preferably used. More concretely,examples are compounds exemplified in U.S. Pat. Nos. 6,228,930,6,071,976, 6,254,990, etc. Examples are dehydration esterificationreactive surface-crosslinking agents include polyalcohol compounds suchas mono-, di-, tri-, or tetra-propylene glycol, 1,3-propanediol,glycerin, 1,4-butanediol, 1,3-butanediol, 1,5-pentanediol,1,6-hexanediol, and sorbitol; epoxy compounds such as ethylene glycoldiglycidyl ether, and glycidol; alkylene carbonate compounds such asethylene carbonate; oxetane compounds; and cyclic urea compounds such as2-imidazolidinone; and the like. One or more kinds of covalentsurface-crosslinking agents (particularly, dehydration reactivesurface-crosslinking agents) can be used in combination. The use amountof the surface-crosslinking agents may be properly determined in a rangeof preferably 0.001 to 10 parts by mass and more preferably 0.01 to 5parts by mass relative to 100 parts by mass of the water-absorbing resinpowder.

(Ion Bonding Surface-Crosslinking Agent)

In the present invention, from the viewpoint of improvement of physicalproperties such as liquid permeability, an inorganicsurface-crosslinking agent can be used other than the organicsurface-crosslinking agent. Examples usable as the inorganicsurface-crosslinking agent is not particularly limited and may includedivalent or higher, preferably, trivalent to tetravalent polyvalentmetal salts (organic salts and inorganic salts) or hydroxides.Concretely, polyvalent metals to be used are aluminum, zirconium, etc.,and aluminum lactate and aluminum sulfate are usable. These inorganicsurface-crosslinking agents may be used simultaneously with orseparately from the organic surface-crosslinking agent. Thesurface-crosslinking with polyvalent metals is exemplified inInternational Publication Nos. 2007/121037, 2008/09843, and 2008/09642,in U.S. Pat. Nos. 7,157,141, 6,605,673, and 6620889, in US PatentApplication Publication Nos. 2005/0288182, 2005/0070671, 2007/0106013,2006/0073969, etc. One or more kinds of ion bonding surface-crosslinkingagents can be used in combination. The use amount may be properlydetermined in a range of preferably 0.001 to 10 parts by mass and morepreferably 0.01 to 5 parts by mass relative to 100 parts by mass of thewater-absorbing resin powder.

In the present invention, from the viewpoint of improvement of physicalproperties such as liquid permeability, a polyamine polymer may be usedsimultaneously or separately in addition to the organicsurface-crosslinking agent. A polyamine polymer having a mass averagemolecular weight of about 5000 to 1000000 is particularly preferablyused, and examples are exemplified in U.S. Pat. No. 7,098,284, WO Nos.2006/082188, 2006/082189, 2006/082197, 2006/111402, 2006/111403,2006/111404, etc.

The present invention can provide a water-absorbing resin with highwhiteness even by high temperature heating or heating treatment with air(hot air) which conventionally causes intense coloration. In the case ofaiming at providing particularly a sanitary material (especially a paperdiaper), it is preferable to improve the absorption capacity under load(AAP) described later to the following range, preferably not less than20 g/g by a surface-crosslinking treatment.

At the time of mixing the surface-crosslinking agent, water ispreferably used as a solvent. The use amount of water may be properlydetermined in a range of preferably 0.5 to 20 parts by mass and morepreferably 0.5 to 10 parts by mass relative to 100 parts by mass of thewater-absorbing resin powder. A hydrophilic organic solvent may be usedin combination if necessary, other than water, and its use amount may beproperly determined in a range of preferably 0 to 10 parts by mass andmore preferably 0 to 5 parts by mass relative to 100 parts by mass ofthe water-absorbing resin powder.

Further, at the time of mixing a surface-crosslinking agent solution, awater-insoluble fine particle powder and a surfactant are allowed tocoexist to an extent that the effect of the present invention is nothindered. The types, the use amount, and the like of the water-insolublefine particle powder and surfactant are described in, for example, U.S.Pat. No. 7,473,739 etc. and the use amount may be properly determined ina range of preferably 0 to 10 parts by mass, more preferably 0 to 5parts by mass, and still more preferably 0 to 1 part by mass relative to100 parts by mass of the water-absorbing resin powder.

(b) Heating Treatment Step

In the surface-crosslinking step, after the water-absorbing resin powderand a surface-crosslinking agent are mixed, the mixture is preferablysubjected to a heating treatment and thereafter to a cooling treatmentif necessary. A conventionally known dryer is used for the heatingtreatment and the above-mentioned stopping method of the presentinvention is preferably applied. The heating temperature (which is aheat medium temperature or a material temperature, particularly amaterial temperature) at the time of the heating treatment may beproperly determined in accordance with the type, amount, and the like ofthe surface-crosslinking agent to be used and it is preferably 70 to300° C., more preferably 120 to 250° C., still more preferably 150 to250° C., and particularly preferably 170 to 230° C. In the case adehydration reactive surface-crosslinking agent is used, the heatingtemperature is preferably 150 to 250° C. and more preferably 170 to 230°C. In the case the treatment temperature is lower than 70° C., theheating treatment time is extended and it results in decrease of theproductivity and further it becomes impossible to form a uniformsurface-crosslinked layer and, therefore, it is not preferable. In thecase the treatment temperature exceeds 300° C., the water-absorbingresin powder is deteriorated and, therefore, it is not preferable. Theheating time for the heating treatment is preferably in a range of 1minute to 2 hours. The heating treatment can be carried out in a commondryer or a heating furnace. The methods of surface-crosslinkingdescribed in EP Patent Nos. 0349240, 0605150, 0450923, 0812873, 0450924,0668080, Japan Patent Application Publication Nos. 7-242709, 7-224304,U.S. Pat. Nos. 5,409,771, 5,597,873, 5,385,983, 5,610,220, 5,633,316,5,674,633, 5,462,972, WO Nos. 99/42494, 99/43720, 99/42496, etc. can beemployed preferably in the present invention.

(Heating Treatment Apparatus)

Various kinds of heating treatment apparatuses such as a fluidized bedheating treatment apparatus, a belt type heating treatment apparatus, apaddle type heating treatment apparatus, a disk type heating treatmentapparatus, a hot air heating treatment apparatus, and an infraredheating treatment apparatus can be used as the heating treatmentapparatus of the present invention. Among them, a paddle type heatingtreatment apparatus is preferable and a disk type heating treatmentapparatus is particularly preferable. Specific examples thereof includea Bepex-heating treatment apparatus and a Nara-heating treatmentapparatus. In this description, for convenience, the term “heatingtreatment apparatus” is employed, but it is same as a dryer. The heatingtreatment can be carried out by heating a jacket or blowing hot air inthe heating treatment apparatus itself. A dryer connected successively,for example, a box type dryer, a rotary tubular furnace, or a heatablescrew is also proper.

The heating temperature in the heating treatment step is preferably 70to 300° C., more preferably 120 to 250° C., still more preferably 150 to250° C., and particularly preferably 170 to 230° C. The heating time forthe heating treatment is preferably in a range of 1 minute to 2 hours,more preferably in a range of 5 minutes to 1 hour.

As the heating treatment apparatus in the heating treatment step, it ispreferable to use a transverse type continuous stirring apparatus havinga charging inlet and a discharge outlet for a water absorbent resinpowder, as well as a stirring means including one or more rotary shaftsequipped with a plurality of stirring disks and a heating means. In thatcase, the stirring power index of the transverse type continuousstirring apparatus is preferable to set to 3 to 15 [W·hr/kg]. Thestirring power index is a value calculated from the following numericalexpression and is an indicator for determining stable productivity ofthe water-absorbing resin in a case of expansion of the scale(particularly, not less than 1 [t/hr]).

(stirring power index)=((power consumption of apparatus at the time ofsurface treatment)−(power consumption at the time of idling)×averageretention time)/(treatment amount per unit time×average retentiontime)  [Mathematic 1]

The stirring power index can be easily calculated from the powerconsumption of apparatus at the time of surface treatment and the powerconsumption at the time of idling, and is more preferably in a range of4 to 13 [W·hr/kg], still more preferably 5 to 11 [W·hr/kg], particularlypreferably 5 to 10 [W·hr/kg], and most preferably 5 to 9 [W·hr/kg]. Ifthis index exceeds 15 [W·hr/kg], the physical properties (particularly,liquid permeability) are deteriorated and, on the other hand, if it isunder 3 [W·hr/kg], the physical properties (particularly, absorptioncapacity under load) are also deteriorated. The stirring power index canbe determined properly in consideration of adjustment of the supplyamount and discharge amount of the water absorbent resin, the particlesize or bulk specific gravity of the water absorbent resin, the rotationspeed and shape of the apparatus, the composition of the surfacetreatment agent, the retention time, and the like.

(Dry Air)

At the time of interruption of the heating treatment step in the presentinvention, from the viewpoint of retention of physical properties of thewater-absorbing resin and suppression of the blocking phenomenon, it ispreferable to inject dry air into the heating treatment apparatus duringthe interruption period of the heating treatment step. The dew point ofthe dry air may be not higher than 0° C. (minus dew point), and ispreferably −100 to −5° C., more preferably −95 to −30° C., still morepreferably −90 to −35° C., and particularly preferably −85 to −40° C. Amethod for controlling the dew point of the air is not particularlylimited and air may be properly dried by using appliances such as amembrane drier, a cooling adsorption type drier, and a diaphragm driersolely or in combination. In the case where a cooling adsorption typedrier is used, it may be of heat regeneration manner, a non-heatregeneration manner, or a non-regeneration manner. To cause the effectof the present invention, it is sufficient that the temperature of airhaving the above-mentioned dew point is room temperature (25° C.). It ispreferable to carry out heating and it is more preferable to carry outheating in a manner that the heating treatment apparatus comes into theabove-mentioned temperature range during the interruption period.

(Conventional Heating Treatment Method)

Conventional surface-crosslinking and a heating treatment of awater-absorbing resin are disclosed in the above-mentioned PatentDocuments 1 to 9 and patent documents described in the paragraphs ofcovalent surface-crosslinking agent and ion bonding surface-crosslinkingagent. However, these documents do not disclose stopping of a heatingtreatment apparatus while keeping the heating treatment apparatus in aheated state.

(c) Cooling Step

This step is a step carried out arbitrarily after the heating treatmentstep. In the case a dehydration reactive crosslinking agent made from apolyhydric alcohol compound, an alkylene carbonate compound, and anoxazolidinone compound, which require high temperature reaction, is usedin the heating treatment step, the cooling step is preferably carriedout.

A cooling apparatus to be used in the cooling step is not particularlylimited and an apparatus having the same configuration as the apparatusused for the heating treatment step can be used. That is, the apparatusmay be the transverse type continuous stirring apparatus or theapparatus described in U.S. Pat. No. 7,378,453 etc. For example, abiaxial stirring apparatus and the like in which cooling water iscirculated in the inner wall or other heat transfer surfaces can beused. The temperature of the cooling water is adjusted to be lower thanthe heating treatment temperature in the surface treatment step and maybe properly determined in a range of preferably not lower than 25° C.and lower than 70° C. Similarly to the case of the heating treatmentapparatus, the stirring power index of the cooling apparatus ispreferably 3 to 15 [W·hr/kg], more preferably 4 to 13 [W·hr/kg], stillmore preferably 5 to 11 [W·hr/kg], particularly preferably 5 to 10[W·hr/kg], and most preferably 5 to 9 [W·hr/kg].

Preferable embodiments of the stirring power index and the cooling stepare disclosed in Japanese Patent Application No. 2009-197063(Application date: Aug. 27, 2009) and Japanese Patent Application No.2009-196967 (Application date: Aug. 27, 2009) and their priorityapplications and the contents of the applications are employed as acontrol method of the stirring power index in the present invention.

(d) Periodical Shielding

From the viewpoint of stability of physical properties and physicalproperty improvement by surface-crosslinking, it is preferable toperiodically shield between the surface-crosslinking step and the secondclassification step. The periodical shielding may be carried out atintervals of preferably 0.001 to 5 minutes, more preferably 0.01 to 1minute, and still more preferably 0.05 to 0.5 minutes. Thewater-absorbing resin is transferred periodically, that is,intermittently (On-Off) between continuous apparatuses (the mixer, theheating treatment apparatus, and if necessary, the cooling apparatus).The periodical shielding may be carried out between a heating apparatusand an arbitrary cooling apparatus (the cooling step).

The periodical shielding is disclosed in JP-A-2009-197022 (Applicationdate: Aug. 27, 2009) and its priority application and the content of theapplication is employed as the periodical shielding in the presentinvention.

(2-6) Fine Powder Recycling Step

The fine powder recycling step in the present invention is a step ofrecycling a fine powder (particularly, a fine powder containingparticles not larger than 150 μm as a main component in an amount ofparticularly not less than 70 mass %) obtained by drying and ifnecessary pulverizing and classification to the polymerization step orthe drying step after separation as it is or after hydration. Forexample, methods described in US Patent Application Publication No.2006/247351, U.S. Pat. No. 6,228,930, etc. can be employed.

The particle size can be controlled by adding the recycled fine powderand additionally, a high solid content, which is indispensable in thepresent invention, can be easily be achieved by adding a water-absorbingresin powder and further, and an addition of the fine powder makesseparation of the water-absorbing resin from a drying belt after dryingeasy and, therefore, it is preferable.

Regarding a water-absorbing resin obtained by a production processinvolving a fine powder recycling step, in a conventional productionprocess, because of uneven drying due to the addition of the finepowder, increase of residual monomers, decrease of absorption capacity,and the like, it has been difficult to obtain a water-absorbing resinwith high physical properties. On the other hand, in the productionprocess of the present invention, particularly in the case the processfor producing a water-absorbing resin involves the fine powder recyclingstep, the process is excellent in the effect of preventing physicalproperty deterioration and preventing coloration. That is, it ispreferable that the solid content of the hydrogel is increased to notless than 45 mass %, not less than 50 mass %, not less than 55 mass %,and not less than 60 mass % by evaporating water or adding awater-absorbing resin fine powder in the polymerization step. Theincrease of the solid content from the monomer (solid content ofhydrogel after polymerization−mass % of monomer before polymerization)is preferably not less than 1 mass %, further 2 to 40 mass %, andparticularly preferably in a range of 3 to 30 mass %. It is alsopreferable to add a step of recycling the water-absorbing resin finepowder after drying or its moisturized product to the polymerizationstep or drying step.

(2-7) Other Steps (and Stopping of Heating)

Besides the above-mentioned continuous steps, if necessary, a surfacetreatment step for a polyvalent metal salt, a recycling step of theevaporated monomer, a granulation step, a fine powder removal step (thesecond classification step), etc. may be provided. In order to cause aneffect for color stabilization with time, prevent gel deterioration, andthe like, the above-mentioned additives may be added to some or all ofthe respective steps if necessary. The production process of the presentinvention preferably involves the fine powder recycling step.

During polymerization or after polymerization, a water-soluble orwater-insoluble polymer, a lubricant, a chelating agent, a deodorant, anantibacterial agent, water, a surfactant, water-insoluble fineparticles, an antioxidant, a reducing agent, etc. may be added and mixedin an amount of 0 to 30 mass % and further about 0.01 to 10 mass % tothe water-absorbing resin particles. These additives may also be used asa surface treatment agent.

One or more steps such as a conveying step, a storage step, a packagingstep, and an addition step of adding other additives (fine particles, adeodorant, an antibacterial agent, etc.) may also be preferably carriedout. These steps are described in Patent Documents 1 to 10 etc. and thestopping after the drying step is carried out in the heated state,preferably at not lower than 50° C. and further not lower than 60° C.The upper limit of the heating temperature is not higher than 140° C.,further not higher than 120° C., and particularly not higher than 100°C., from the viewpoint of the cost and coloration. The above-mentionedminus dew point is also preferably employed. The stopping of these stepsmeans a state where an apparatus is unused, that is, a state where thewater-absorbing resin is substantially absent and the water-absorbingresin is not supplied to and discharged out of an apparatus in eachstep.

That is, in the present invention, the surface treatment step(especially the surface-crosslinking step) is stopped in a heated state;preferably, the drying step and the surface treatment step (especiallythe surface-crosslinking step) are stopped in a heated state; and morepreferably, other steps (especially all steps) after the surfacetreatment step in addition to the drying step and the surface treatmentstep (especially the surface-crosslinking step) are stopped in a heatedstate. It is made possible to restart operation without any trouble andcarry out stable operation by stopping in the heated state.

[3] PHYSICAL PROPERTIES OF WATER-ABSORBING RESIN

The water-absorbing resin of the present invention contains apolyacrylic acid (salt)-type water-absorbing resin as a main componentand obtained by the above-mentioned polymerization method,surface-crosslinking method, and the like, in the case where thewater-absorbing resin is used for sanitary goods, particularly paperdiapers. The obtained water-absorbing resin is preferable to control atleast one of the following (3-1) to (3-6), further two or more includingAAP, and particularly three or more. In the case the water-absorbingresin does not satisfy the following respective physical properties,there is a possibility that high concentration diapers containing notless than 40 mass % of the water-absorbing resin cannot sufficientlyexhibit the performance.

(3-1) CRC (Absorption Capacity without Load)

The CRC (absorption capacity without load) of the water-absorbing resinto be obtained by the present invention is preferably 10 [g/g] orhigher, more preferably 20 [g/g] or higher, still more preferably 25[g/g] or higher, and particularly preferably 30 [g/g] or higher. Theupper limit of the CRC is not particularly limited, is preferably 50[g/g] or lower, more preferably 45 [g/g] or lower, still more preferably40 [g/g] or lower. In the case the CRC is lower than 10 [g/g], the waterabsorption amount of the water-absorbing resin is small and thewater-absorbing resin is possibly unsuitable for use for an absorbentbody in sanitary goods such as paper diapers. In the case the CRCexceeds 50 [g/g], if the water-absorbing resin is used for such anabsorbent body, it may possibly become impossible to obtain sanitarygoods excellent in liquid intake speed and, therefore, it is notpreferable. The CRC can be properly controlled by the above-mentionedinternal crosslinking agent, surface-crosslinking agent, and the like.

(3-2) AAP (Absorption Capacity Under Load)

The AAP (absorption capacity under load) of the water-absorbing resin tobe obtained by the present invention is preferably not lower than 20[g/g], more preferably not lower than 22 [g/g], and still morepreferably not lower than 24 [g/g] at a pressure of 4.83 kPa (0.7 psi)by employing the heating treatment as an accomplishing means in order toprevent leakage in paper diapers. The upper limit of the AAP is notparticularly limited; however, from the viewpoint of balance with otherphysical properties, it is preferably not higher than 40 [g/g]. In thecase the AAP is lower than 20 [g/g], if the water-absorbing resin isused for an absorbent body, it may possibly become impossible to obtainsanitary goods with little liquid return (generally, also referred to as“Re-Wet”) at the time a pressure is applied to the absorbent body and itis not preferable. The AAP can be properly controlled by theabove-mentioned surface-crosslinking agent, the particle size, etc.

(3-3) SFC (Saline Flow Conductivity)

The SFC (saline flow conductivity) of the water-absorbing resin to beobtained by the present invention, which is a liquid permeability underpressure, is preferably 1 [×10⁻⁷·cm³·s·g⁻¹] or higher, more preferably10 [×10⁻⁷·cm³·s·g⁻¹] or higher, still more preferably 50[×10⁻⁷·cm³·s·g⁻¹] or higher, particularly preferably 70[×10⁻⁷·cm³·s·g⁻¹] or higher, and most preferably 100 [×10⁻⁷·cm³·s·g⁻¹]or higher by employing the heating treatment as an accomplishing meansin order to prevent leakage in paper diapers. The upper limit of the SFCis not particularly limited; however, from the viewpoint of balance withother physical properties, it is preferably not higher than 3000[×10⁻⁷·cm³·s·g⁻¹], more preferably not higher than 2000[×10⁻⁷·cm³·s·g⁻¹]. In the case the SFC exceeds 3000 [×10⁻⁷·cm³·s·g⁻¹],if the water-absorbing resin is used for such an absorber, liquidleakage may possibly occur in the absorber and, therefore, it is notpreferable. The SFC can be properly controlled by the above-mentioneddrying method and the like.

(3-4) EXT (Amount of Water Soluble Components)

The EXT (amount of water soluble components) of the water-absorbingresin to be obtained by the present invention is preferably 35 mass % orlower, more preferably 25 mass % or lower, still more preferably 15 mass% or lower, and particularly preferably 10 mass % or lower. In the casethe Ext exceeds 35 mass %, the water-absorbing resin to be obtained maypossibly become weak in the gel strength and inferior in the liquidpermeability. If such a water-absorbing resin is used for an absorber,it may possibly become impossible to obtain a water-absorbing resin withlittle liquid return (re-wet) at the time a pressure is applied to theabsorber and it is not preferable. The Ext can be properly controlled bythe above-mentioned internal crosslinking agent and the like.

(3-5) Residual Monomer

The residual monomer amount of the water-absorbing resin to be obtainedby the present invention, from the viewpoint of safety, is controlled tobe preferably 0 to 400 ppm, more preferably 0 to 300 ppm, still morepreferably 0 to 200 ppm. The residual monomer amount is controlled bythe above-mentioned polymerization method and the like.

(3-6) Initial Color Hue

The water-absorbing resin to be obtained by the present invention isexcellent in the initial color hue. The color hue (initial color hue) ofa water-absorbing resin immediately after production obtained by thepresent invention shows the following numerical value. The initial colorhue means the color hue immediately after production and in general, itis the color hue measured before shipment from a plant. It is also avalue measured within 1 year from the production in the case ofpreservation in an atmosphere at not higher than 30° C. and a relativehumidity of not higher than 50% RH. Concretely, in a Hunter Lab system,the L value (Lightness) is preferably not lower than 85, more preferablynot lower than 87, and still more preferably not lower than 89. The bvalue is preferably −5 to 10, more preferably −5 to 9, still morepreferably −4 to 8, and particularly preferably −1 to 7. The a value ispreferably −2 to 2, more preferably −1 to 1, still more preferably −0.5to 1, and particularly preferably 0 to 1. As another chromaticity, YI(Yellow Index) value is preferably not higher than 10, more preferablynot higher than 8, and particularly preferably not higher than 6. Asanother chromaticity, the WB (White Balance) value is preferably notlower than 70, more preferably not lower than 75, and particularlypreferably not lower than 77. The water-absorbing resin obtained by thepresent invention is excellent in coloration with time and showssufficient whiteness in an accelerating test carried out in hightemperature and high humidity.

[4] USE OF WATER-ABSORBING RESIN

Use of the water-absorbing resin obtained by the production process ofthe present invention is not particularly limited and thewater-absorbing resin may be used for absorbing articles includingsanitary goods such as paper diapers, sanitary napkins, and pads forincontinent, water retention agents for agriculture and horticulture,solidifying agents for waste liquids, industrial water shieldingmaterials, and the like.

EXAMPLES

Hereinafter, the present invention will be described in detail withreference to examples and comparative examples; however, the presentinvention should not be construed in a limited way based on thedescription of the examples. For convenience, “litter” is expressed as“L” and “mass %” as “wt %” in some cases. The various physicalproperties described in CLAIMS and Examples of a water-absorbing resinobtained by the present invention were measured according to EDANAmethod and the following measurement examples under the condition ofroom temperature (20 to 25° C.) and a humidity of 50% RH unlessotherwise specified.

1. Initial Color Hue and Coloration with Time

The initial color hue and coloration with time of a water-absorbingresin of the present invention were measured according to measurementmethods described in WO No. 2009/005114 pamphlet.

2. Resin Solid Content (Solid Content)

A water-absorbing resin in an amount of 1.00 g was weighed in analuminum cup with a bottom surface diameter of about 50 mm and the totalweight W1 [g] of the sample (the water-absorbing resin and the aluminumcup) was accurately weighed.

Next, the sample was stood still in an oven at an atmosphere temperatureof 180° C. and the water-absorbing resin was dried. After 3 hours, thesample in the aluminum cap was taken out of the oven and cooled to roomtemperature in a desiccator. Thereafter, the total weight W2 [g] of thesample (the water-absorbing resin and the aluminum cup) after drying wasmeasured and the solid content (unit; [mass %]) was calculated accordingto the following expression.

Solid content[wt %]=100−{(W1−W2)/(weight of water-absorbingresin[g])×100}  [Mathematic 2]

In the case of measuring the resin solid content of a particulatedwater-containing gel-like crosslinked polymer (particulated hydrogel),the sampling amount of the particulated hydrogel was changed to be 2 to4 g and the drying time was changed to be 24 hours.

3. SFC (Saline Flow Conductivity)

the SFC (saline flow conductivity) of the water-absorbing resin to beobtained by the present invention was measured according to thedescription in the specification of U.S. Pat. No. 5,669,894.

4. Other Physical Properties

The physical properties of a water-absorbing resin to be obtained by thepresent invention, that is, CRC (absorption capacity without load),particle size distribution, pH soluble matter, remaining acrylic acidamount, and the like, were measured according to ERT of EDANA or thedescription in the specification of US Patent Application PublicationNo. 2006/204755.

Production Example 1

A water-absorbing resin (A) was produced by employing continuousproduction apparatuses for carrying out a continuous polymerizationstep, a drying step by a band dryer, a pulverizing step, aclassification step, a surface treatment step (wetting and mixing step,heating step, and cooling step), a particle regulation step, and aconveying step for joining the respective steps.

That is, 0.04 g of sodium persulfate (to 1 mol of monomer) and 100 ppmof diethylenetriamine pentaacetic acid pentasodium salt werecontinuously mixed by line mixing with an aqueous solution of acrylicacid sodium salt partially neutralized in an amount of 70 mol % (monomerconcentration: 53 mass %) containing 0.03 mol % (based on monomer) ofpolyethylene glycol diacrylate (average molecular weight: 478) as aninternal crosslinking agent and the mixture was supplied to a beltpolymerization apparatus and subjected to aqueous solutionpolymerization. The hydrogel obtained in this manner was crushed by avertical crusher (manufactured by Orient Co., Ltd., screen of 12 mm) toobtain a particulated water-containing gel-like polymer (A) containing70 mass % of solid content and having fluidity.

The particulated water-containing gel-like polymer (A) was continuouslylayered by using a traverse feeder for a continuous ventilating belttype dryer on a punching metal of a continuous ventilating belt whichwas continuously operated by controlling the sequence of the traversefeeder and continuously dried on the ventilating belt for 35 minutes.The discharge amount (treatment amount) of the dried material was 1.7[t/hr]. The dying apparatus and drying condition were as shown in thefollowing (a) to (c).

(a) Belt Type Dryer

A dryer used was a continuous ventilating dryer having 6 drying chambersin total which can independently adjust the hot air temperature andwhich have the same size and in which a ventilating belt passes. Thehydrogel passed in about 5.8 minutes (=35 min/6 chambers on the belt) inthe respective 6 drying chambers.

(b) Hot Air Temperature and Linear Velocity

The hot air temperature of the drying chambers was heated to be 180° C.by steam and the linear velocity of the hot air was set to be 1.6 m/s.The blow direction of the first chamber was upward from the bottomsurface and the blow direction of the second to the sixth chambers wasset downward to the bottom surface from an upper part of the dryer.

(c) Ventilating Belt

The ventilating belt used was a stainless steel belt (band) made of SUS304 and having long round holes with a width of 1.2 mm and a length of15 mm in a zigzag arrangement and a porosity of 27%. The particulatedwater-containing gel-like polymer (A) was agglomerated like blocks afterdrying. After loosened into several millimeters, the entire amount ofthe loosened dried product was continuously supplied to a roll mill(roll gaps: 1.0 mm/0.55 mm/0.42 mm from the upper side) and pulverizedand classified with a sieving apparatus having a metal sieving net withmeshes of 850 μm to obtain a water-absorbing resin powder (A).

The water-absorbing resin powder (A) was continuously supplied to a highspeed continuous mixing apparatus (Turbulizer; 1000 rpm) by pneumatictransportation (temperature 35° C. and dew point −15° C.) from theclassifying apparatus at 1.5 t/hr and at the same time a surfacetreatment agent solution (1) was mixed by spraying (wetting and mixingstep). The surface treatment agent solution (1) was a mixed solutioncontaining 1,4-butanediol, propylene glycol, and pure water. The surfacetreatment agent solution (1) was mixed with the water-absorbing resinpowder (A) at a ratio of 0.3 parts by mass of 1,4-butanediol, 0.5 partsby mass of propylene glycol, and 2.7 parts by mass of pure waterrelative to 100 parts by mass of the water-absorbing resin powder (A) togive a mixture (A), a wet powder.

The obtained mixture (A) was then surface-treated by a transverse typecontinuous stirring apparatus (1) (paddle type heating treatmentapparatus) having a downward inclined angle of 1°, an aspect ratio of2.2, a paddle rotation speed of 13 rpm, scraping blades, two rotaryshafts having a surface roughness (Rz) of the inner surface of 500 nm(heating treatment step). At that time, the inside of the apparatus (1)was suctioned by a suctioning gas discharge apparatus having a bagfilter, and the inside pressure of the apparatus was reduced to 1 kPa. Arotary valve (periodically shielding apparatus) was installed in theinlet and outlet of the apparatus (1). According to a previous test, theposition of a discharge bank which gave an average retention time ofabout 45 minutes and an average filling ratio of 75% was obtained, andthe discharge bank was set at the position as obtained. A heating sourceused for the surface treatment was pressurized steam at 2.5 MPa, and theatmosphere temperature in the apparatus was measured by a thermometerinstalled near the discharge part of the transverse type continuousstirring apparatus (1), and the steam flow rate was controlled to carryout the heating in such a manner that the temperature was adjusted to198° C. The total surface area of the stirring disks and the stirringshafts was 24.4 m² and the mass surface area ratio calculated from thetotal surface area (heat transfer surface area) and the treatment amountwas 61.5 [kg/m²/hr]. The stirring power at the time of the surfacetreatment was 27.8 kW, the stirring power in idling was 13.5 kW, theaverage retention time was 45 minutes, and the stirring power index was9.5 [W·hr/kg]. Using same transverse type continuous stirring apparatusso that the water absorbent resin was forcibly cooled to 60° C. (coolingstep).

The water-absorbing resin (A) which was a adjusted substance all under850 μm was obtained by carrying out classification by a sievingapparatus to separate the substance under 850 μm, and mixing thesubstance on 850 μm (substance that did not pass through 850 μm) afterpulverized again with the substance under 850 μm. The physicalproperties of the water-absorbing resin (A) are shown in Table 2.

Comparative Example 1

After the continuous polymerization, the continuous drying, thecontinuous pulverizing classification, and the continuous surfacetreatment in the Production Example 1 were carried out continuously for20 days, operation of all of the apparatuses including the heating ofthe paddle type heating treatment apparatus (transverse type continuousstirring apparatus (198° C.) was stopped for 48 hours in order to changethe product (change the internal crosslinking agent). After 48 hours,the atmosphere temperature in the heating treatment apparatus wasincreased from room temperature (25° C.) to 198° C. and then thecontinuous operation was restarted to continuously produce awater-absorbing resin. In this case, a colored foreign matter was mixedinto the water-absorbing resin at the beginning of the restartedoperation and an excess load (increase of power consumption) on thepaddle type heating treatment apparatus (transverse type continuousstirring apparatus) was found at the time of restarting of theoperation.

Example 1

In Comparative Example 1, the atmosphere temperature was changed to be120° C. during the time of stopping the paddle type heating treatmentapparatus (transverse type continuous stirring apparatus). That is, theoperation was carried out in the same manner as in Comparative Example1, except that at the time of stopping of the continuous production, theheating temperature (the atmosphere temperature measured by athermometer installed near the discharge part) of the stopped paddletype heating treatment apparatus (transverse type continuous stirringapparatus) was controlled to be 120° C. from 180° C. by adjusting thesteam flow rate and the atmosphere temperature in the inside of theheating treatment apparatus was increased to be 198° C. from 120° C.after 48 hours from the stopping of the step to restart the continuousoperation. When a water-absorbing resin with 0.1 mol % of the internalcrosslinking agent was continuously produced, no foreign matter wasgenerated at the beginning of operation and no excess load (increase ofpower consumption) was found on the transverse type continuous stirringapparatus.

Example 2

In Example 1, the operation was carried out in the same manner as inExample 1, except that dry air with a dew point of −30° C. was injectedto the heating treatment apparatus at the time of stopping of the paddletype heating treatment apparatus (transverse type continuous stirringapparatus). No foreign matter was generated at the beginning ofoperation and no excess load (increase of power consumption) was foundat the time of restarting of the heating treatment apparatus.

The operation conditions and occurrence of malfunctions in ComparativeExample 1 and Examples 1 and 2 are collectively shown in the followingTable 1.

TABLE 1 Temperature Period in the of Excess Stopped stopping stoppingload at Abnormal Foreign apparatus period [° C.] [hr] restarting noisematter Comparative Paddle type Room 48 Found Generated Generated Example1 heating Temperature treatment apparatus Example 1 ↑ 120 ↑ None NoneNone Example 2 ↑ 120 ↑ None None None

Comparative Example 2

In Production Example 1, on the 10th day after the continuous operationwas started, because of a trouble in a cooling apparatus installeddownstream of the paddle type heating treatment apparatus, operations ofall of the apparatuses including heating (198° C.) were stopped whilethe water-absorbing resin powder was left in the inside of the heatingtreatment apparatus (transverse type continuous stirring apparatus).When the continuous operation was restarted and the water-absorbingresin was continuously produced after the atmosphere temperature in theinside of the heating treatment apparatus was increased from roomtemperature (25° C.) to 198° C. after 48 hours, foreign matters weremixed at the beginning of operation and an excess load (increase ofpower consumption) was found at the time of restarting of the operationof the heating treatment apparatus (transverse type continuous stirringapparatus). The substance on 850 μm (substance that did not pass through850 μm) was increased. Table 2 shows the physical properties of thecomparative water-absorbing resin (2) 5 hours after the restarting ofthe operation.

Example 3

In Comparative Example 2, the temperature was changed to be 120° C.during the time of stopping the paddle type heating treatment apparatus(transverse type continuous stirring apparatus). That is, the sameoperation as the operation in Comparative Example 2 was carried out,except that at the time of stopping continuous production, the heatingtemperature of the heating treatment apparatus (transverse typecontinuous stirring apparatus) was lowered to 120° C. from 198° C. Whenthe continuous operation was restarted and the water-absorbing resin wascontinuously produced after the atmosphere temperature in the inside ofthe heating treatment apparatus was increased from 120° C. to 198° C.after 48 hours, no foreign matter was generated at the beginning of theoperation and no excess load (increase of power consumption) was foundat the time of restarting of the operation of the heating treatmentapparatus (transverse type continuous stirring apparatus). Table 2 showsthe physical properties of the water-absorbing resin (3) as a product 5hours after the restarting of the operation.

Example 4

In Example 3, the temperature was changed to be 198° C. during the timeof stopping the paddle type heating treatment apparatus (transverse typecontinuous stirring apparatus). That is, the same operation as theoperation in Example 3 was carried out, except that at the time ofstopping continuous production, the heating temperature of the heatingtreatment apparatus (transverse type continuous stirring apparatus) waskept to be 198° C. as it was. When the continuous operation wasrestarted and the water-absorbing resin was continuously produced at theatmosphere temperature in the inside of the heating treatment apparatusof 198° C. after 48 hours, no foreign matter was generated at thebeginning of the operation and no excess load (increase of powerconsumption) was found at the time of restarting of the operation of theheating treatment apparatus (transverse type continuous stirringapparatus). However, the water-absorbing resin in the inside of theheating treatment apparatus was yellowed. Table 2 shows the physicalproperties of the water-absorbing resin (4) as a product 5 hours afterthe restarting of the operation.

Production Example 2

In Production Example 1, a water-absorbing resin (B) was produced bycarrying out the same operation as the operation in Production Example1, except that a surface treatment agent solution (2) obtained byadditionally adding 1 part by mass of an aqueous solution of 27%aluminum sulfate to the surface treatment agent solution (1) was used.The physical properties of the obtained water-absorbing resin (B) areshown in Table 2.

Comparative Example 3

In Production Example 2, on the 10th day after the continuous operationwas started, because of a trouble in a cooling apparatus installeddownstream of the paddle type heating treatment apparatus, operations ofall of the apparatuses including heating (198° C.) were stopped whilethe water-absorbing resin powder was left in the inside of the heatingtreatment apparatus (transverse type continuous stirring apparatus).When the continuous operation was restarted and the water-absorbingresin was continuously produced after the atmosphere temperature in theinside of the heating treatment apparatus was increased from roomtemperature (25° C.) to 198° C. after 48 hours, foreign matters weremixed at the beginning of operation and an excess load (increase ofpower consumption) was found at the time of restarting of the operationof the heating treatment apparatus (transverse type continuous stirringapparatus). The substance on 850 μm (substance that did not pass through850 μm) was increased. Table 2 shows the physical properties of thecomparative water-absorbing resin (3) 5 hours after the restarting ofthe operation.

Example 5

In Comparative Example 3, the temperature was changed to be 120° C.during the time of stopping the paddle type heating treatment apparatus(transverse type continuous stirring apparatus). That is, the sameoperation as the operation in Comparative Example 3 was carried out,except that at the time of stopping continuous production, the heatingtemperature of the heating treatment apparatus (transverse typecontinuous stirring apparatus) was lowered to 120° C. from 198° C. Whenthe continuous operation was restarted and the water-absorbing resin wascontinuously produced after the atmosphere temperature in the inside ofthe heating treatment apparatus was increased from 120° C. to 198° C.after 48 hours, no foreign matter was generated at the beginning of theoperation and no excess load (increase of power consumption) was foundat the time of restarting of the operation of the heating treatmentapparatus (transverse type continuous stirring apparatus). Table 2 showsthe physical properties of the water-absorbing resin (5) as a product 5hours after the restarting of the operation.

Production Example 3

In Production Example 1, a water-absorbing resin (C) was produced bycarrying out the same operation as the operation in Production Example1, except that a surface treatment agent solution (3) consisting of 1.0part by mass of ethylene carbonate and 3.0 parts by mass of pure waterwas used in place of the surface treatment agent solution (1) and theaverage retention time in the heating treatment apparatus was changed tobe 50 minutes. The physical properties of the water-absorbing resin (C)are shown in Table 2.

Comparative Example 4

In Production Example 3, on the 20th day after the continuous operationwas started, because of a trouble in a cooling apparatus installeddownstream of the paddle type heating treatment apparatus, operations ofall of the apparatuses including heating (198° C.) were stopped whilethe water-absorbing resin powder was left in the inside of the heatingtreatment apparatus (transverse type continuous stirring apparatus).When the continuous operation was restarted and the water-absorbingresin was continuously produced after the atmosphere temperature in theinside of the heating treatment apparatus was increased from roomtemperature (25° C.) to 198° C. after 60 hours, foreign matters weremixed at the beginning of operation and an excess load (increase ofpower consumption) was found at the time of restarting of the operationof the heating treatment apparatus (transverse type continuous stirringapparatus). The substance on 850 μm (substance that did not pass through850 μm) was increased. Table 2 shows the physical properties of thecomparative water-absorbing resin (4) 5 hours after the restarting ofthe operation.

Example 6

In Comparative Example 4, the temperature was changed to be 120° C.during the time of stopping the paddle type heating treatment apparatus(transverse type continuous stirring apparatus). That is, the sameoperation as the operation in Comparative Example 4 was carried out,except that at the time of stopping continuous production, the heatingtemperature of the heating treatment apparatus (transverse typecontinuous stirring apparatus) was lowered to 120° C. from 198° C. Whenthe continuous operation was restarted and the water-absorbing resin wascontinuously produced after the atmosphere temperature in the inside ofthe heating treatment apparatus was increased from 120° C. to 198° C.after 60 hours, no foreign matter was generated at the beginning of theoperation and no excess load (increase of power consumption) was foundat the time of restarting of the operation of the heating treatmentapparatus (transverse type continuous stirring apparatus). Table 2 showsthe physical properties of the water-absorbing resin (6) as a product 5hours after the restarting of the operation.

TABLE 2 Absorption Absorption Saline flow capacity without capacityunder conductivity load CRC load AAP SFC [g/g] [g/g] [×10⁻⁷ · cm³ · s ·g⁻¹] Production Water-absorbing Steady 26.5 23.3 70 Example 1 resin (A)operation Comparative Comparative 5 hours after 26.8 23.1 55 Example 2water-absorbing restarting of resin (2) the operation Example 3Water-absorbing 5 hours after 26.4 23.2 68 resin (3) restarting of theoperation Example 4 Water-absorbing 5 hours after 26.1 22.9 70 resin (4)restarting of the operation Production Water-absorbing Steady 26.6 22.6107 Example 2 resin (B) operation Comparative Comparative 5 hours after26.6 22.3 81 Example 3 water-absorbing restarting of resin (3) theoperation Example 5 Water-absorbing 5 hours after 26.6 22.6 105 resin(5) restarting of the operation Production Water-absorbing Steady 26.623.0 73 Example 3 resin (C) operation Comparative Comparative 5 hoursafter 27.0 22.7 61 Example 4 water-absorbing restarting of resin (4) theoperation Example 6 Water-absorbing 5 hours after 26.5 23.0 72 resin (6)restarting of the operation

As shown in Table 1 and Table 2, deterioration of the physicalproperties (particularly saline flow conductivity) of thewater-absorbing resin after restarting of the operation was observed inComparative Example 2 (temperature at the time of stopping was roomtemperature) and it took 1 to several days until the physical propertieswere stabilized. On the other hand, substantially no deterioration ofthe physical properties at the time of restarting of the operation wasobserved by keeping the heating state during the stopping as in Example3 (the temperature at the time of stopping was 120° C.) and Example 4(the temperature at the time of stopping was 190° C.) and thereafterrestarting the operation. This effect was confirmed in the case wherethe surface-crosslinking agents were changed (comparison betweenComparative Example 3 and Example 5 and comparison between ComparativeExample 4 and Example 6).

In the production process of the present invention, the physicalproperties can be quickly stabilized after restarting of the operationby keeping the apparatus in a heated state during the interruption ofthe operation and, therefore, it is made possible to carry outcontinuous production (preferably for not shorter than 10 days) of awater-absorbing resin in a huge scale (preferably not less than 1[t/hr]). The conventional production processes described in PatentDocuments 1 to 9 and in patent documents described in the paragraphs ofthe covalent surface-crosslinking agent and ion bondingsurface-crosslinking agent do not at all imply the stopping method andits effect of the present invention.

INDUSTRIAL APPLICABILITY

A water-absorbing resin free from coloration and a foreign matter can beproduced economically and stably by continuous production in a hugescale (e.g., 1 [t/hr] or more) and the water-absorbing resin of thepresent invention can be used for various kinds of sanitary materialssuch as paper diapers and sanitary napkins and also for variousapplications.

1. A process for producing a water-absorbing resin comprising apolymerization step of polymerizing an aqueous unsaturated monomersolution, a drying step of drying a particulated water-containinggel-like crosslinked polymer obtained in a finely crushing step duringthe polymerization or after the polymerization, a pulverizing step afterthe drying, a classification step after the drying, and a surfacetreatment step for the water-absorbing resin powder after theclassification step, wherein the surface treatment step is interruptedwith a heating treatment apparatus kept in a heated state and thereafterthe surface treatment step is restarted, and the interruption of thesurface treatment step means a state where the water-absorbing resinpowder is substantially absent in the heating treatment apparatus, ornot charged to or not discharged out of the heating treatment apparatusin the continuous surface treatment.
 2. The production process accordingto claim 1, wherein the surface treatment step is surface-crosslinkingby a heating reaction.
 3. The production process according to claim 1,wherein the heating treatment apparatus is a transverse type continuousstirring apparatus having a charging inlet and a discharge outlet for awater absorbent resin powder, as well as a stirring means including oneor more of rotary shafts equipped with a plurality of stirring disks anda heating means, and the apparatus is stopped in a heated state.
 4. Theproduction process according to claim 1, wherein the interruption timeof the surface treatment step is not shorter than 0.5 hours and within100 days.
 5. The production process according to claim 1, wherein thetemperature of the heating treatment apparatus during the interruptiontime of the surface treatment step is lower than the heating treatmenttemperature by 10° C. or more.
 6. The production process according toclaim 1, wherein the temperature of the heating treatment apparatusduring the interruption time of the heating treatment step is 80 to 140°C.
 7. The production process according to claim 1, wherein thewater-absorbing resin fine particles obtained in the classification stepare recycled to the particulated water-containing gel-like crosslinkedpolymer before the drying.
 8. The production process according to claim1, wherein the water-absorbing resin powder is subjected to the surfacetreatment at not less than 1 [t/hr] in the surface treatment step. 9.The production process according to claim 1, wherein heating of theheating treatment apparatus during the interruption time of the surfacetreatment step is carried out by heated steam or hot air.
 10. Theproduction process according to claim 9, wherein the heated steam isrecycled.
 11. The production process according to claim 1, furthercomprising, after the surface treatment step, a second classificationstep after the surface-crosslinking, and/or a fine powder recovery stepafter the classification step.
 12. The production process according toclaim 1, wherein a gas having a dew point of −100 to −5° C. is injectedinto the heating treatment apparatus during the interruption time of thesurface treatment step.
 13. The production process according to claim 1,wherein the surface treatment step is continuous surface treatment fornot shorter than 24 hours.
 14. A process for producing a water-absorbingresin comprising a polymerization step of polymerizing an aqueousunsaturated monomer solution, a drying step of drying a particulatedwater-containing gel-like crosslinked polymer obtained in a finelycrushing step during the polymerization or after the polymerization, apulverizing step after the drying, a classification step after thedrying, and a surface treatment step for the water-absorbing resinpowder after the classification step, wherein the surface treatment stepis interrupted and heating of a heating treatment apparatus is stopped,then cleaning of the inside of the heating treatment apparatus isstarted within 100 hours and thereafter the surface treatment step isrestarted, and the interruption of the surface treatment step means astate where the water-absorbing resin powder is substantially absent inthe heating treatment apparatus, or is not charged to or not dischargedout of the heating treatment apparatus in the continuous surfacetreatment.
 15. The production process according to claim 1, wherein asurface-crosslinking agent to be used in the surface treatment step is adehydration reactive surface-crosslinking agent.
 16. The productionprocess according to claim 14, wherein a surface-crosslinking agent tobe used in the surface treatment step is a dehydration reactivesurface-crosslinking agent.